AVS1996 Session MI+NS-MoA: Magnetic Imaging

Monday, October 14, 1996 1:30 PM in Room 106A/B

Monday Afternoon

Start	Invited?	Item
1:30 PM	Invited	MI+NS-MoA-1 Magnetic Force Microscopy and Micromagnetics R. Proksch, S. Foss, G. Skidmore, C. Merton, J. Schmidt, E. Dahlberg (University of Minnesota) Magnetic force microscopes (MFMs) have been used to map magnetic field variations with submicron resolution. The interactions of the magnetic probe with the stray micromagnetic fields from the sample are mapped to form an image that yields qualitative information about the micromagnetic structure [1] of the sample. Quantification has been more problematic. We have used the magnetic fields of magnetotactic bacteria (MTB) [2] to compare with MFM models for a quantification of the magnetic structure (MTB produce intracellular chains of permanent single domain particles of magnetite (Fe3O4) to orient themselves using the terrestrial field thus aiding their search for the most amenable environment). This quantification compared favorably with bulk measurements of the bacteria magnetic moments. The result being the measurement of a single magnetic moment of order 10-13 emu. The development of this quantification process allows micromagnetic models of fundamental magnetic structures such as the domain walls in the archetypal ferromagnet iron and the archetypal ferrimagnet magnetite to be critically tested [3,4]. [1] E. Dan Dahlberg and Jian-Gian Zhu, Physics Today 48, 34 April 1995.[2] R. B. Proksch, B. M. Moskowitz, E. D. Dahlberg, T. Schaeffer, D. A. Bazylinski and R. B. Frankel, Appl. Phys. Lett. 66, 2582 (1995).[3] Roger Proksch, Sheryl Foss, E. Dan Dahlberg, and Gary Prinz, J. Appl. Phys. 75, 5776 (1994).[4] R. Proksch, S. Foss, and E. Dan Dahlberg, IEEE Trans. Mag. 30, 4467 (1994).
2:10 PM		MI+NS-MoA-3 Direct Comparison of Magnetic Imaging using SEMPA and MFM M. Kelley, D. Tulchinsky, R. Celotta (National Institute of Standards & Technology) Scanning Electron Microscopy with Polarization Analysis (SEMPA) and Magnetic Force Microscopy (MFM) have proven extremely useful for observing and studying the magnetic domain structure of microscopic magnetic systems. Each technique has unique strengths which can often be exploited for specific types of measurements. Unfortunately each technique also has disadvantages which may limit its potential for some measurement problems. For example, the advantage MFM enjoys in wide availability and ease of use is offset by complications of tip/sample interactions and the difficulty of extracting quantitative information about sample magnetization from MFM data. On the other hand, SEMPA provides a direct measurement of sample magnetization and has high spatial resolution, but has stringent requirements on specimen preparation and slower sample throughput. Because of these differences, the two measurement techniques are often complementary, each providing information which is critical to the development of a correct understanding of the sample under investigation.We have begun an effort to make a detailed comparison between these two techniques. The goals of this effort are to highlight the relative strengths and weaknesses of these measurement techniques and to exploit the quantitative high resolution capabilities of SEMPA to provide a detailed characterization of the sensitivity and resolution of MFM. Preliminary results will be presented of SEMPA and MFM images of high density magnetic recording media.This work was supported in part by the Office of Naval Research.
2:30 PM		MI+NS-MoA-4 Cross-Tie Walls on the Surface of Permalloy Films Studied by SEMPA Y. Lee, A. Kueny, A. Koymen (University of Texas, Arlington) Scanning Electron Microscopy with Polarization Analysis(SEMPA) was used to image the surface magnetic domain structure of permalloy film (350\Ao\, Ni\sub 83\Fe\sub 17\) in ultra-high vacuum. The samples were demagnetized along the easy axis by an alternating magnetic field with decreasing amplitude. Using the high resolution SEMPA system zig-zag domain walls, which were separating two large approximately head-on domains were observed. Along the straight edges of the zig-zag domain walls, cross-tie walls and Bloch lines were observed with a periodic vortex structure. The cross-tie walls occurred at the points where the magnetization was reversed by 180 degree across the straight edge of the wall.
2:50 PM		MI+NS-MoA-5 A High-Resolution Photoemission Electron Microscope for Chemical and Magnetic Imaging, and Micro-Spectroscopy K. Grzelakowski (Focus GmbH, Germany); G. Schoenhense, W. Swiech (University of Mainz, Germany); O. Froemter, J. Kirschner (MPI fuer Mikrostrukturphysik, Germany); M. Sander (Omicron Vakuumphysik GmbH, Germany) The capabilities of a new generation multi-purpose Photoemission Electron Microscope instrument with integral sample stage (FOCUS IS-PEEM) for surface sensitive and spectroscopic microscopy will be demonstrated by results on the magnetic domain structure and chemical composition of Permalloy on Si, magnetite on MgO, and Fe-whiskers. The IS-PEEM is designed to exploit the advantages of synchrotron radiation, but can also be used with laboratory sources. A lateral resolution down to 40 nm (depending on the sample) is made possible by high-precision electron optics with stigmator/deflector, and in-situ variable contrast and iris apertures. Photon excitation energies ranging from UV to X-rays allow micro-spectroscopy with minimum sample damage, in contrast to SEM. Element specific PEEM images the lateral distribution of a specific element using electron emission excited by photon energies tuned to an absorption edge of this element. Chemical contrast can be observed on-line with the PEEM, or with enhanced contrast after background subtraction of a second image taken directly below the edge; the integral sample stage of the PEEM avoids mechanical influences (drift or vibrations) on the image quality. In micro spot X-ray absorption spectroscopy and Micro-spot-ESCA (XPS, UPS, AES) the region of interest is selected using the PEEMs variable iris aperture; local X-ray absorption or ESCA spectra from areas down to below 1 micron are then measured using an integral analyser. Magnetic domain imaging is based on the magnetic circular dichroism (MCD), which gives rise to a strong contrast in the PEEM. Magnetic and element-specific imaging can be performed simultaneously.
3:10 PM	Invited	MI+NS-MoA-6 Quantitative Magnetic Force Microscopy on Ferromagnetic and Superconducting Materials H. Hug (University of Basel, Switzerland); G. Bochi (Massachusetts Institute of Technology); B. Stiefel, A. Moser, H. G\um u\ntherodt (University of Basel, Switzerland); R. O'Handley (Massachusetts Institute of Technology) Since its invention Magnetic Force Microscopy (MFM) has become a widely used tool to image and modify the domain structure of ferromagnetic materials, to map the field or to test the local response of recording heads and to image and modify vortices in superconductors. However it has been difficult to extract quantitative information on the local magnetization structure of the sample from MFM data. After a general introduction into MFM image formation, the application of the Magnetic Force Microscope to the Cu/NiCu/Si(001)-system and to the YBa\sub 2\Cu\sub 3\O\sub 7-x\-system is discussed. The Cu/NiCu/Si(001) has been well studied by both, new MFM techniques and more conventional measurement techniques sensitive to magnetism and structure. The combination of these techniques and theoretical modeling gives new insight into the complex dependence of the magnetization on film thickness and external field. On the YBa\sub 2\Cu\sub 3\O\sub 7-x\-system the combination of low temperature MFM and theoretical modeling using the Ginsburg-Landau theory allows the determination of the local penetration depth from vortex images. Further, future applications and possibilities of MFM will be discussed.
3:50 PM		MI+NS-MoA-8 Wrinkled Magnetization in Ultrathin Ferromagnetic Films T. Duden (Technische Universitaet Clausthal, Germany); E. Bauer (Arizona State University) Using spin-polarized low energy electron microscopy (SPLEEM) we have found that ultrathin cobalt films grown epitaxially on W(110) are not purely in-plane magnetized but have over a wide thickness range (3-8 monolayers) also a significant perpendicular magnetization component whose magnitude decreases with increasing thickness. The regions with constant perpendicular magnetization component have a length scale which is very different from that of the in-plane component. This leads to a wrinkled magnetization with domain configurations which are strongly influenced by steps. The phenomenon is a consequence of the competition between the anisotropies of the film/substrate interface and of the free surface. Changing these anisotropies by Au undercoatings and overcoatings causes major changes in the wrinkle pattern which are the main subject of the talk.
4:10 PM		MI+NS-MoA-9 Low Temperature Magnetic Force Microscopy of Vortices in YBa\sub 2\Cu\sub 3\O\sub 7\\sub -\\sub x\ Thin Films B. Stiefel, H. Hug, A. Moser, O. Fritz, H. G\um u\ntherodt (University of Basel, Switzerland) Due to the extremely short coherence length grain boundaries play an important role in high-TC-superconductors. It has been shown early that grain boundaries act as weak links in polycrystalline materials. Our low temperature magnetic force microscopy study of a laser ablated YBCO thin film reveals that the boundaries between the growth islands play a key-role for the pinning of vortices: All vortices in low external magnetic fields are pinned in thes grain-boundaries. The comparison of the measured field distribution of a single vortex with our theoretical models gives local information about the strength of the superconducting order parameter. Further we use the magnetic force microscope to modify the arrangement of vortices in both, thin films and single crystals. In thin films we nucleate a vortex bundle and observe its movement due to an external current. In single crystals some vortices can be pushed out of their pinning centers individually. The measurement of the pinning force of single vortices seems feasible.
4:30 PM		MI+NS-MoA-10 Magnetic Depth Profiling and Domain Imaging of Patterned Magnetic Films using Scanning Ion Microscopy with Polarization Analysis (SIMPA) G. Steierl, Z. Zhou, Carl Rau (Rice University) Scanning Ion Microscopy with Polarization Analysis (SIMPA) is a novel technique for the magnetic analyis of ferromagnetic ultra-thin films and magnetic multilayers. We report on recent experiments that show the power of this new method and point towards the wide range of applications. In SIMPA, a highly focused 20keV Ga\super +\ - ion beam (minimum spot size: 35nm) is scanned over a sample and induces the emission of spin-polarized secondary electrons. Spin analysis of these electrons gives a direct measurement of the surface domain pattern. At high beam intensities, SIMPA allows for layer-by-layer removal, and a magnetic depth profile of the magnetization can be recorded. We report on domain-images and well-defined depth profiles of the magnetization obtained from Fe/Mn/Fe/Pd(100) trilayers. From extensive magnetic depth-profiling experiments, we find, for the first time, an interrelation between the various top- and bottom Fe layer domain patterns. The SIMPA microscope (current resolution =150nm) can also be used to fabricate magnetic structures confined in two dimensions. We report on recent SIMPA results obtained from a variety of arrays of magnetic permalloy structures which were also studied by using Diffraction Magneto-Optic Kerr Effect (DMOKE). \super *\ Supported by the National Science Foudation and the Welch Foundation.